Cinematical has the story for this trailer. Short version: Disney and Pixar were looking to go their separate ways, but Disney owned the rights to everything Pixar had ever made. Disney created its own animiation division to develop sequals to Pixar films, like Monsters, Inc. Disney and Pixar got over their issues, that new divison went away, but this video of a non-Pixar sequel to Monsters, Inc. remains.

Watch the "trailer", then head over to Cinematical to find out the details.